Background: Limited data indicate that obese children are absent from school more than their normal-weight peers. We analyzed administrative data from a large urban school district to investigate the association of obesity and student sociodemographic characteristics with absenteeism. Methods: We analyzed 291,040 records, representing 165,056 unique students (grades 1-12). Obesity status was classified according to Centers for Disease Control and Prevention age- and sex-specific percentiles for body mass index (BMI) and analyses were based on negative binomial regression. Results: Overall rates of overweight and obesity were 17% and 20%, respectively, and the estimated absence rate was 17 absences per 180 student-days. Obesity was weakly associated with increased school absences. The association was present mainly among the most obese students (BMI greater than 99th percentile), who had an 11% greater absence rate compared to normal-weight students. Compared to white students, Hispanics and African Americans had higher absence rates (14% and 10%, respectively), and Asians had lower absence rates (43%). Students eligible for free or reduced-cost meals had 24% higher absence rates than those who were not eligible. Conclusions: Overweight and obesity do not seem strongly associated with school absence, except among extremely obese children. Race and poverty appear to affect absences to a greater extent than overweight and obesity. Additional research is needed to investigate the contribution of contextual factors in schools and neighborhoods. This study suggests that data routinely collected in schools could be used to track childhood obesity and to efficiently evaluate public health interventions designed to decrease childhood obesity. (Contains 1 table.)